1. Field of the Invention
The present invention relates to a method for correcting speed feed-back in a synchronous permanent-magnet motor.
2. Description of the Background Art
The problem is that, in the prior art, speed feedback in a synchronous permanent-magnet motor changes slowly e.g. as a function of temperature. When used as an elevator drive machine, the permanent-magnet motor is typically subjected to long-lasting peek-level loads, during which the temperature of the elevator machine rises. As the machine is developing heat, the speed feedback sensor attached to the machine becomes heated. In the present context, the above-mentioned speed feedback sensor is a tachometer. Thus, the speed information obtained from the tachometer changes, e.g. drifts, as a function of the temperature of the machine and especially of the tachometer. In the above-mentioned situation, the speed feedback information typically includes a 3% gain and zero error, which is visible in the entire speed regulation system. A rise in the machine temperature may naturally also be due to development of heat in the elevator shaft when a synchronous permanent-magnet motor is used as an elevator drive motor.
Sensor drift is well known to mean that a sensor output has some error value that varies (drifts) for some reasons, e.g. time, temperature, utilization etc. FIGS. 3a and 3b show an effect of offset drift and gain drift on a sensor output signal. In case of tachometers, offset drift and gain drift are caused, for example, when temperature changes because effective mechanical dimensions of tachometer changes causing drift in output signal, or effective magnetic fields inside tachometer changes causing drift in output signal. Offset drift and gain drift are also caused, for example, when using a tachometer (after installation) because tachometers have brushes (like DC motors), which will wear (mechanically) the more the tachometer is used. Wearing causes changes in electrical contacts and therefore, there will also be a drift in tachometer output signal. If the tachometer is not used regularly, contact elements will become oxidized causing drift. Such wearing effects may take weeks, months, even years in regular elevator use.
When using the tachometer as a servomotor feedback sensor, for example, in elevator systems, there may be up to a 10% error (drift) in a tachometer output signal, mainly because of tachometer gain drift. Both offset drift and gain drift may increase or decrease the tachometer signal (positive or negative drift), this behavior depends on tachometer constructions, manufacturing processes, usage etc. If drift errors are not compensated, the elevator may run slower than designed, it will have problems for stopping at floor stops, there may be oscillations in an elevator drive motor rotation speed, etc.
Previously known solutions attempted in order to deal with the above-mentioned problem by generating an advance estimate of the speed signal error produced in the above-described manner and eliminating the error on an average. However, this method would not lead to accurate and reliable correction of the error.
Another known way of solving the aforesaid problem is based on measurement of speed feedback. In this case, a known distance is traversed at a known speed, so the speed feedback error can be corrected at the operating point in question. The problem with this method is the continuously changing operating point, so the correction is naturally inaccurate.